Here are links to the release notes and download link for the CU's released.
NAV 2013 – Cumulative Update 56
You can download the cumulative update from KB 4052191
NAV 2013 R2 – Cumulative Update 49
You can download the cumulative update from KB 4052192
NAV 2015 – Cumulative Update 37
You can download the cumulative update from KB 4052193
NAV 2016 – Cumulative Update 25
You can download the cumulative update from KB 4052194
NAV 2017 – Cumulative Update 12
You can download the cumulative update from KB 4052195